Apple Scab: About and significance of early season management

Article written by Madeline Wimmer, UMN Extension Fruit Production Educator

 

Images: Apple scab can cause a wide range of damage on various apple types based on their inherent resistance, the amount of spores present in the spring, along with weather conditions and orchard management programs. Damage can be seen in the above pictures as late-season apple scab lesions and chlorosis (yellowing) on a crabapple leaf (left), and minor apple scab infection on a ripe apple right. 

As the season continues, if you think you've observed apple scab or want to report a severe infection year for your orchard, feel welcome to reach out to me by filling out our UMN Extension Fruit Crop Observation Form. 

Apple scab is considered to be the most common disease of apples in Minnesota. It can be mild to severe in how it impacts production, depending on how much action is taken to manage and prevent infections.  

How pathogen biology and life cycle impact orchard management choices

The disease-causing organism is the fungal pathogen Venturia inaequalis, which is a part of the ascomycota: a classification of fungi that produce tiny, spore-producing fruiting bodies, which can be somewhat thought of as microscopic mushrooms. When present, these fruiting bodies can overwinter primarily on leaves, which then provide spores (i.e., primary spores) in the spring that can infect new leaves and flowers when conditions are warm and wet for a long enough time (i.e., leaf wetness duration).  

The stage at which these spores are being released from overwintering fruiting bodies leads to lesion symptoms on leaves and fruits called primary infections. The process of primary spores being discharged can generally last until 1-2 weeks after petal fall. 

It can also be estimated using degree day (DD) accumulations starting at the green tip date, where most primary spores will be discharged by 400-600 DD, depending on weather conditions and other factors. The combination of the green tip date, tracking rain events, and degree days, is how infection models like NEWA begin estimating percent primary spore maturity and cumulative discharge throughout the season. 

If leaves or fruits are infected, and conidia are formed, these spores can go on to cause what is known as secondary infections throughout the rest of the growing season. During very wet years with little fungicide use, conidia can rapidly multiply and this can lead to extreme infections where tree defoliation can occur. 

Thus, early season management that eliminates primary spores from infecting leaves and apple fruits is advantageous to keeping apple scab infections under control, especially in orchards with previous-season infections. Orchards in these circumstances may want to adhere to a more attentive management program this year. 

Understanding the likelihood for apple scab infection events to occur

In the field of plant diseases, the concept of the disease triangle is essential to understanding pathogen risk development and likelihoods for infection. The disease triangle includes: 1) pathogens, 2) environment, and 3) host susceptibility. 

Pathogens are the disease-causing organisms, while the environment reflects the surrounding conditions that influence pathogen development, spore release, and the rate of infection. Finally, host susceptibility impacts how easily a pathogen can infect and continue to infect plant(s). 

When apple scab is present in an orchard, primary spore release and subsequent infections can happen after rain events and during wet conditions that have adequate humidity and warmth. This means there can also be delays in how long it takes for primary spores to fully discharge when springs are cooler in Minnesota.  

Below is a table originally produced by Mills and La Plante that compares the amount of time it takes in hours for primary spores to cause an infection at a given temperature 

Table 2. Apple scab likelihood for infection based on variations in temperature* 

Table originally created by Mills and La Plante. An accessible version of this table may be found on the UC IPM webpage Apple Scab.

This table can help estimate the risk or likelihood of primary and secondary apple scab infection, and help guide fungicide timing of applications. However, because it assumes continuous leaf wetness, the Mills-La Plante table may not fully reflect real-world conditions, which can lead to reduced accuracy in certain situations. For the purposes of this article, it can help you understand how increases in ambient temperatures can decrease the amount of time it takes for infection to occur during wet periods. 

Preventative practices

Encouraging leaf decomposition or removal: If you think your orchard is at high risk for infection this year, collecting and flail mowing, or removing last year’s leaves is an orchard sanitation practice that can help encourage leaf decomposition and reduce risk for apple scab infection occurrence. This practice can be done in the fall or the spring before green tip begins. 

For mowing, it’s recommended to blow leaves into the aisle, or gather them by some other means. In the fall, it can also help to apply urea as a 5% solution to the apple leaves on the orchard floor to further encourage decomposition. 

Choosing resistant varieties: Certain apple varieties have some resistance to apple scab, which includes Honeycrisp. Resistance is based on a single or multiple apple genes that help either stop or slow down apple scab disease development. 

One of UMN’s most recent apple releases, Triumph is considered more resistant to apple scab than other varieties like Honeycrisp, making it a potential option for growers with limited spray options. One way to increase apple scab resilience for new orchards, or new orchard plantings is to choose resistant apple varieties. Keep in mind that a resistant variety is not fully immune to the disease and some amount of infection can still occur. 

This information can usually be found through nurseries suppliers and through fact sheets online, or you can refer to the UMN Extension web page about apple scab under the tab, “Apple scab resistance of Minnesota hardy apple varieties.”

To continue learning more about apple scab, visit our UMN Extension webpage, “Apple scab of apples and crabapples.”

For more information on chemical management for apple scab, visit the Midwest Fruit Pest Management Guide starting on page 14.